Coal fired power plants generate fly ash from the combustion of pulverized coal, or combinations of pulverized coal and other carbonaceous supplemental fuels such as petroleum coke, woodbark, charcoal, wood, residual fibers, etc. The fly ash is captured in the power plant""s emission control devices, such as by electrostatic precipitators and baghouses. The principal composition of fly ash includes an inert mineral fraction consisting primarily of ferro-alumino-silicate glass and residual unburned carbon from coal and/or other supplemental fuels.
The inert mineral fraction of fly ash is a pozzolan, which makes fly ash an acceptable mineral admixture for use in Portland cement concrete. The fly ash itself includes the devolatized mineral matter which has been trapped or loosely associated with the coal as well as incombustible components and elements of the coal and/or of the supplemental fuels. In addition to such incombustible components, the fly ash contains carbon rich particles which have not been completely combusted, usually due to the inefficiency of the boiler design or related conditions.
Specifications for the use of fly ash in Portland cement concrete are set out in ASTM #C-618. This specification limits the loss-on-ignition (LOI) content of fly ash pozzolan to less than 6%. The LOI value of fly ash in generally equal to the percent by weight of the unburned carbon content of the fly ash. This carbon content can vary from as little as about 0.5% up to 20% or more of the weight of the total fly ash product. However, for pozzolanic activity, a good quality fly ash should contain less than 1% carbon but, in any case, no more than 4% carbon.
A high carbon fly ash as a pozzolanic mixture has a detrimental impact upon the quality of concrete. The presence of carbon reduces air entrainment, which, in many locations, is the only real protection that concrete has against freeze-thaw/wet-dry conditions. Therefore, the lower the carbon content, the better the concrete mix is from an air entrainment perspective. The presence of carbon also increases water requirements, reduces pozzolanic reactivity, and degrades the appearance of finished concrete surface. Thus, carbon negatively affects the strength, durability, and aesthetic appearance of concrete. Therefore, the lower the carbon content in any fly ash, the better the fly ash as a concrete admixture.
In many cases, air entrainment agents (AEA) are added to the concrete mix in order to desirably increase the amount of small air bubbles that are formed during mixing. In these cases, carbon reduces the ability of these agents to operate efficiently.
The present invention is directed to a chemical treatment that may be applied to the high carbon content fly ash either prior to or concurrent with its mixing with the other concrete components.
Specifically, aromatic carboxylic acid or hydroxy substituted aromatic carboxylic acid is added to a quantity of fly ash to reduce air entraining chemical demand. Such aromatic carboxylic acids include benzoic, phthalic, isophthalic, and terephthalic acids and their salts. Included in the hydroxy substituted aromatic carboxylic acids found effective are salicylic acid, m-hydroxybenzoic acid, and p-hydroxybenzoic acid and their salts.